As of patch 2.3 all items in the gladiator season 1, 2, and 3 sets will be part of the same set, so you can use those items interchangeably and keep the set bonuses (ie: you can wear the season 2 shoulders along with the season 3 helm, chest, legs) and retain the 4 piece set bonus.

The separate sets for separate specs are going to stay the same, but now the set bonuses for each spec will be upgraded to work the same as the new 2 and 4-piece bonuses that are coming out, regardless of season. So for example, your class and spec's dps set bonus will work no matter which season your arena dps gear comes from. If I were an arena gladiator, I would be jumping up and down for joy right now. As it is, I still think that's pretty neat.

[edited for clarity -- the set bonuses will work between seasons, but not between different specs, so healing sets and dps sets won't combine together. Also, the arena season 1 gear will still be moved into the honor system once season 3 comes out]